An X-ray CT (Computed Tomography) scanner is a device for reconstructing an image by using a system for processing data having a difference in an X-ray attenuation rate (X-ray absorption coefficient) inside an object. The X-ray CT scanner includes an X-ray source that irradiates an object with X-rays and an X-ray detector that detects X-rays transmitted through the object. The object is interposed between the X-ray source and the X-ray detector, and they face each other. They rotate around the object while keeping a position in which the object is interposed therebetween and they face each other. Then, X-ray transmission image data of the object is scanned from a plurality of projection directions. Usually, it is possible for the X-ray source of the X-ray CT scanner to use an X-ray tube in which an anode is irradiated with electrons accelerated using a high voltage to emit X-rays. In addition, the X-ray detector has a structure having X-ray detection elements arranged like a two-dimensional array so as to quickly scan a broad area at one scan.
Projection data as obtained by a scan using the X-ray CT scanner include not only information on the intensity of X-rays (primary X-rays) transmitted through the object without scattering, but also information on the incident intensity of X-rays (scattered X-rays) scattered in the object, etc. In order to remove the scattered X-rays, the X-ray CT scanner has an anti-scatter grid to remove the scatted X-ray generated in the object and the grid is positioned at the X-ray source side of the X-ray detector. Even this method, however, cannot remove all the scattered X-rays. Because of this, certain software is also used to perform scatter correction (e.g., see the following Patent Literatures 1 to 3).